Accurate measurement of the zero-field splitting (ZFS) is important in the development of paramagnetic contrast agents (PCA) used in magnetic resonance imaging. The ZFS controls the electron relaxation rate which affects the proton relaxivity; it also reveals important relationship between the PCA chelate structure and the PCA's effectiveness. Whereas previous ZFS measurements of gadolinium contrast agents consisted of indirect methods of measuring the EPR linewidth, we obtain their ZFS parameters, D and E, by characterizing their powder spectra at several EPR frequencies. The EPR powder spectra of frozen glasses of GdCl3, GdDTPA, GdEDTA, GdTTHA, and GdDOTA at L-band, X-band, and Q-band (1.5 GHz, 9.5 GHz, and 35 GHz) are measured. Varying the frequency controls the relative magnitude of the ZFS and the Zeeman interactions, resulting in changes in the powder lines. These are interpreted with the aid of a spectral simulation program for an S = 7/2 powder spectrum.